Purification of manganese electrolyte solutions



Patenteid Apr. 20,

PURIFICATION OF MANGANESE ELECTRO- LYTE SOLUTIONS ,g V PaulM. Ambrose, College Park, Md., assignor to ChicagoDevelopment Company, Chlcago,-I ll., a .7

corporation of Illinois 7 No Drawing. Application February 11, 1939, 'serialNo. 255,877

1 Claim.

This invention relates to the electrolytic deposition of manganese and is particularly concerned with improvements in the process of purifying the electrolyte solutions which are employed in the deposition processes.

It has already been proposed to purify electrolytes for the preparation of electrolytic manganese, which electrolytes comprise aqueous solutions of manganese salts such as manganese sulphate and manganese chloride. Some of these methods have not been entirely satisfactory due to their relative complexity, the difflculty of obtaining the desired removal or impurities, the cost involved, and for otherreasons with which those skilled in the art are iamiliar. I

In the usual practice of electrolytically depositing manganese, the electrolyte employed contains, in addition to the soluble manganese salt such as manganese sulphate or manganese chloride, a substance which forms a complex salt with manganese to prevent or inhibit the precipitation of manganous hydroxide until the hydrogen ion concentration has reached a point definitely on the alkaline side such as at a pH of 7.5 to 9.0. In Patent No. 2,119,560, the substance is an ammonium salt such as ammonium sulphate. In the copending application of Reginald S. man, Ser. No. 219,163, filed July 14, 1939, there is disclosed a variety of susbstances of organic character and of a type which form complex salts with inanganese including, among others, hydroxylamine,

monoethanolamine, and triethanolamine.

Electrolysis is carried out in such solutions in various ways so that the solutions either become more acid or slightly more alkaline as the electrolysis proceeds. It is substantially uniformly desirable, however, to'start with a slightly alkaline electrolyte. Catholyte solutions which are employed in the production of electrolytic manganese usually contain relatively large amounts of manganese salts and ammonium salts or salts of organic nitrogenous bases or the like, an illustrative example being of the order of 70 grams of manganese sulphate and 200 grams of ammonium sulphate per liter as major constituents. However, the electrolyte also usuall contains small amounts of iron, nickel, cobalt, molybde num, vanadium, arsenic and lead. These impurities seriously interfere with the production of satisfactory deposits of high-purity electrolytic manganese.

In accordance with the present invention, the purification oi. the electrolyte is carried out in an efiective, expeditious manner, at relatively low 'cost, and in a manner which blends in excellently with the facilities available in an electrolytic manganese plant.

The practice of the present invention brings about the removal from the manganese electrolyte of metal ions more noble than manganese by the electrodeposition of these elements. In other words, the manganese electrolyte is electrolyzed' under such conditions that the metallic impurities more noble than manganese are irreversibly deposited on the cathode and the concentration of the manganese in the electrolyte is substantially unaffected. In general, this is accomplished by the utilization of relatively low cathode current densities so that the same are insufficient to maintain a deposit of manganese on the cathode when other metallic impurities more noble than manganese are present in the electrolyte. "It is thought that, under the conditions of operation of the present process, the manganese is deposited on the cathode but that it is redissolved in the electrolyte substantially as rapidly as it is deposited. Whatever the mechanism of the process may be. however, it has been demonstrated that the purification of the electrolyte is effected without appreciable and in most cases without detectable decrease of the concentration of manganese in the electrolyte.

Incarrying out the process of the present invention, the iron which may be present in the electrolyte is preferably preliminarily removed by the known method of oxidizing it to the ferric state by the addition of manganese dioxide to the electrolyte at. a pH'above the neutral point. The precipitated ferric hydroxide is then filtered oil. The manganese dioxide also serves to oxidize molybdenum and vanadium, which may be present, to their state of maximum valence although it does not precipitate these two elements. The molybdenum and vanadium, if present as molybdates or vanadates, respectively, may be removed by adding a solution of a soluble lead salt. as disclosed in the copending application of Reginald S. Dean, Serial No. 249,377, filed January 5, 1939. This precipitates the molybdenum and vanadium as lead molybdate and lead vanaxiate, respectively,

which may be removed by filtration. As disclosed in the aforesaid application of Reginald'S. Dean, Serial No. 249,377, the soluble lead salt is added prior to filtering oil the ferric hydroxide so that the filtration of the same and the lead mclybdate and lead 'vanadate may be carried out in the sam step.

The resulting electrolyte solution is then ready for the step of electrolysis to eifect the deposition and, therefore, removal of the nickel, co-

bait. lead or whatever other metals more noble than manganese may be present in the electro- Mac. The electrolysis is best carried out in a diaphragm cell in which the cathodes and anodes are separated by a material only slightly pervious to the solution to be electrolyzed. Heavy canvas and alundum have both been used successfully for this purpose although any material having generally similar characteristics tor the present purpose may be substituted therefor. Since the solution to be electrolyzed must be in contact with the electrodes for several minutes.- the cathode compartments should preferably be larger than those ordinarily used in the electrodeposition of manganese metal. A convenient size of compartment for use in a small plant is 26 inches long, 3 inches wide, and 48 inches deep. I! twenty compartments of this size are used in the purification cell, each cell can purify enough manganese sulphate solution each day for the production of a ton of manganese metal in an electrolytic manganese plant.

The cathodes may be of any suitable metal such as iron or steel. The anodes may also be made of a variety of metals although excellent results have been obtained with leadanodes. The shape of the electrodes is relatively unimportant. It has been found, however, that the larger the cathode surface the faster the deposition and also the purer willbe the solution at the outlet point of the cell. Wire gauze, crumpled foil, coils, woven springs and the like are, therefore, recommended as forms which the cathode may take since they have been found to be superior to fiat sheets.

The anode and cathode compartments are preferably alternated in the purification cell and the solution to, be electrolyzed is preferably fed into the cathode compartment at or near the top thereof and isremoved in any desirable way. Preferably, the floor of the cathode compartment is perforated or comprises a false bottom which leads into a jacket in part surrounding the wall of the compartment. The treated electrolyte is then conveniently removed near the top of the jacket portion.

The anode and cathode current densities are subject to some variation but, in general, the cathode current density is a minor fraction of that of the anode current density. As has been pointed out hereinabove, the cathode current density must be suiliciently low so that a manganese deposit will not be maintained on the cathode. A satisfactory anode current density is about 20 amperes per square foot and, in such case, the cathode current density may be in the neighborhood of 2 amperes per square foot. Those skilled in the art will be able to select conditions of operations to obtain good results in the light of the teachings herein.

The anolyte surrounding the cathode compartment or compartments is an aqueous solution of an electrolyte. A water solution of 200 grams of ammonium sulphate and 30 grams of sulphuric acid per liter has given excellent results. The concentration of ammonium sulphate and sulphuric acid may, however, be substantially less or substantially greater. In place of ammonium sulphate and sulphuric acid, any electrolyte, preferably acid in character, may be employed. Among the metals more noble than manganese which are removed from the manganese solution by electrolysis in accordance with the present inention are tellurium, zinc, chromium, gallium, iron, cadmium, indium, thallium, cobalt, nickel,

tin, lead, antimony, bismuth, arsenic, copper, silver, mercury, platinum, and gold.

The following examples are illustrative of the practice of the invention. It will be understood that variations may be made with respect to the proportions of metals, the nature of the cathodes and anodes, current densities and th like as well as in other ways all within the scope of the invention in the light of the guiding principles which have been described in detail hereinabove.

Example 1 200 milliliters of catholyte, containing 200 grams ofammonium sulphate and 70 grams of manganese sulphate per liter. at a pH of 7.52, from which the iron and the molybdenum had previously been removed but which still contained traces of lead, arsenic, nickel and cobalt, were electrolyzed for 20 minutes in a glass cell using an alundum cup as a diaphragm, platinum foil as a cathode, and sheet lead as an anode. The anode current density was 18 amperes per square foot. At the end of the electrolysis, the solution was free of contaminants.

Example 2 500 milliliters of catholyte, containing approximately 190 grams of ammonium sulphate and 70 grams of manganese sulphate per liter having a a pH of 7.36, from which the iron and molybdenum had previously been removed but which contained traces of lead, arsenic, nickel and cobalt, were electrolyzed for 20 minutes in a lead-lined cell using a heavy canvas diaphragm, interwoven iron wire coils as a cathode and a lead anode. The anode current density was approximately 18 amperes per square foot. At the end of the electrolysis the solution was free of contaminants.

Example 3 20 gallons of catholyte, containing approximately 200 grams of ammonium sulphate and 70 lons per hour and the outlet solution was free of contaminants.

What I claim as new and desire to protect by Letters Patent of the United States is:

A method of purifying manganese electrolyte solutions to remove at least one metal which is more noble than manganese. which comprises providing an anode compartment containing an anode and a cathode compartment containing a cathode, separating said compartments by a material only slightly pervious to the electrolyte, and electrolyzing the manganese electrolyte for a period of time sufficient to effect the purification to the desired extent. the cathode current density being insuflicient to maintain a deposit of manganese on the cathode in the presence of more noble metals, the ratio of the cathode current density to the anode current density being about 1 to 10.

PAUL M. AMBROSE. 

